The use of radio communication networks is rapidly becoming a part of the daily life for more and more people around the globe. For instance, the GSM (Global System for Mobile Communications) networks offer a variety of functions. Generally, radio communication systems based on such networks use radio signals transmitted by a base station in the downlink over the traffic and control channels are received by mobile or portable radio communication terminals, each of which have at least one antenna. Historically, portable terminals have employed a number of different types of antennas to receive and transmit signals over the air interface. For example, monopole antennas mounted perpendicularly to a conducting surface have been found to provide good radiation characteristics, desirable drive point impedances and relatively simple construction. Monopole antennas can be created in various physical forms. For example, rod or whip antennas have frequently been used in conjunction with portable terminals. For high frequency applications where an antenna's length is to be minimized, another choice is the helical antenna. In addition, mobile terminal manufacturers encounter a constant demand for smaller and smaller terminals. This demand for miniaturization is combined with desire for additional functionality such as having the ability to use the terminal at different frequency bands, e.g. of different cellular systems, so that a user of the mobile terminal may use a single, small radio communication terminal in different parts of the world having cellular networks operating according to different standards at different frequencies.
Further, it is commercially desirable to offer portable terminals, which are capable of operating in widely different frequency bands, e.g., bands located in the 800 MHz, 900 MHz, 1800 MHz, 1900 MHz and 2.0 GHz regions. Accordingly, antennas, which provide adequate gain and bandwidth in a plurality of these frequency bands will need to be employed in portable terminals. Several attempts have been made to create such antennas.
In order to reduce the size of the portable radio terminals, built-in antennas have been implemented over the last couple of years. The general desire today is to have an antenna, which is positioned inside the housing of a mobile communication terminal. The most common built-in antennas currently in use in mobile phones are the so-called planar inverted-F antennas (PIFA). This name has been adopted due to the fact that the antenna looks like the letter F tilted 90 degrees in profile. Such an antenna needs a feeding point as well as a ground connection. If one or several parasitic elements are included nearby, they can be either coupled to ground or dielectrically separated from ground. The height of the PIFA antennas is often a limiting factor for decreasing the size of the mobile communication terminal. The geometry of a conventional PIFA antenna includes a radiating element, a feeding pin for the radiating element, a ground pin for the radiating element, and a ground substrate commonly arranged on a printed circuit board (PCB). Both the feeding pin and the ground pin are necessary for the operation of such an antenna, and are arranged perpendicular to the ground plane, wherein the PIFA radiating element is suspended above the ground plane in such a manner that the ground plane covers the area under the radiating element. This type of antenna, however, generally has a fairly small bandwidth in the order of 7% of the operating frequency. In order to increase the bandwidth for an antenna of this design, the vertical distance between the radiating element and the PCB ground may be increased, i.e. the height at which the radiating element is placed above the PCB is increased. This, however, is an undesirable modification as the height increase makes the antenna unattractive for small communication devices and may reduce directivity. One solution to this problem is to add a dielectric element between the antenna and the PCB, in order to make the electrical distance longer than the physical distance. U.S. Pat. No. 6,326,921 to Ying et al discloses a built-in, low-profile antenna with an inverted planar inverted F-type (PIFA) antenna and a meandering parasitic element, and having a wide bandwidth to facilitate communications within a plurality of frequency bands. A main element is placed at a predetermined height above a substrate of a communication device and the parasitic element is placed on the same substrate as the main antenna element and is grounded at one end. The feeding pin of the PIFA is proximal to the ground pin of the parasitic element. The coupling of the meandering, parasitic element to the main antenna results in two resonances, which are adjusted to be adjacent to each other in order to realize a broader resonance encompassing the DCS (Digital Cross-Connect System), PCS (Personal Communications System) and UMTS (Universal Mobile Telephone System) frequency ranges. However, prior art antenna designs will still be a limiting factor when developing radio terminals with adequate bandwidth to cover, for example, all of the DCS, PCS and UMTS frequency bands, at the same time recognizing the desire to provide compact terminals.
The known solutions have mainly dual band performance, e.g. EGSM+DCS, or triple band performance. However, both GSM and EGSM (EGSM is an acronym for Extended Global System for Mobile communications—Extended GSM) are generally not achievable by the prior art antenna solutions fulfilling the above mentioned spatial requirements, i.e. known antennas of the discussed type are not capable of operating efficiently in both the GSM 850 MHz and the EGSM 900 MHz bands.
US-A1-2005/0110692 discloses a multi-band radio antenna device having a flat ground substrate, a flat main radiating element, and flat parasitic elements separated from the main radiating element and connected to ground. The main radiating element is located adjacent to and in the same plane as the flat ground substrate. This planar requirement restrains the design possibilities of the radiating element, which must be oriented in the same plane as the ground substrate, i.e. the antenna is limited to flat, planar implementations. Furthermore, this antenna device necessitates a plurality of separated individual elements besides the radiating element, including the parasitic elements, which each need an individual contact. Moreover, the efficiency of this antenna should be improved, e.g. in order to enhance battery life of a mobile communication terminal using such an antenna device.
Most existing solutions use a ¼ wave for the high-band configuration, as the aforementioned antenna device of US-A1-2005/0110692.
EP-A-1 263 079 discloses an antenna comprising a driven element and a parasitic element resonant at different frequencies so that the antenna has a bandwidth encompassing both resonant frequencies. A second driven element, resonant at a third frequency, may be added so that the antenna is also usable in a third different separate band. This element may also be in the form of a meander. However, the shorter radiating element of the antenna arrangement is at least partly shaped into acute angles in zigzag and used as ¼ wave radiating element for the high-band. Further, the radiating elements are placed near the feeding point.
US 2003/210188 A1 discloses a multi-band antenna system including a retractable whip antenna and a meander antenna having a plurality of selectively coupled meander elements formed on a dielectric flexible board. However, this antenna system is not related to compact built-in antennas devised to be incorporated into mobile or portable radio communication terminal.
WO 99/56345 A discloses a multi-band antenna device comprising a plate element, on which at least two antenna elements intended for transmitting and receiving are formed. They have a common feeding point. The shorter radiating element of the antenna arrangement is at least partly shaped into acute angles in zigzag and used as ¼ wave radiating element for the high-band. Further, the radiating elements are placed near the feeding point.
Other known solutions are variable pitch meanders have been used in the past on stub antennas to achieve dual-band performance, but are generally difficult to tune and cannot be used more generally in PIFA configurations.
More specifically, these prior art antennas generally rely on ¼ wave elements to form the primary resonances in the high-bands. In certain cases, the antenna can be designed such that there are significant currents on the high-band as well as the low-band elements. This tends to improve the high-band efficiency and bandwidth significantly. However, ½ wave elements for the 1800 band were up to now not implemented due to the space requirements. This generally means that the PCS efficiency of known antennas differs from their DCS efficiency, typically it is 1-2 dB higher. Also, because it is common to use two resonances in the high-band, a significant amount of tuning is required to center these resonances around 50 Ohms in order to achieve optimum gain.
A more general problem with known built-in antennas is not only small bandwidth, but also significantly worse gain performance than a traditional external antenna i.e. some kind of stub antenna.
Furthermore electrical contacts are expensive, at least with regard to mass produced products, such as mobile communication terminals. As mentioned above, the PIFA antenna type needs at least two contacts, and often even more contacts for the additional parasitic elements. Hence, it would be advantageous to minimize the number of contacts that a multi-band radio antenna device needs for assembly in a mobile communication terminal.
Hence, an improved multi-band radio antenna device would be advantageous and in particular a multi-band radio antenna device allowing for increased efficiency with regard to e.g. size, cost, bandwidth, design flexibility and/or energy consumption of the multi-band radio antenna device would be advantageous.